I always put third-party content libraries in the personal prefs folder location, because using the location under "/Applications/MAXON/*" can lead to ACL access issues. If you put the .lib4d in the library/browser dir in the main C4D application dir, perhaps try putting it in the prefs folder location instead?

I've tried placing the .lib4d file in the library/browser accessible through clicking the preferences button within C4D, but I get the same result as the OP. I've also tried pasting the .lib4d file to the other library/browser folder but the result is the same. I really have no idea at this point on how to get these material previews to show up!

---Edit---

Is there perhaps an issue with the included .lib4d file?
I compared this install process with other material packs I've installed, namely GSG and Pixel Lab packs, and a few others for VRay, and noticed that the process is effectively the same as what has been recommended for successfully installing the Cycles pack. However, and unfortunately, I've still not been successful in getting this dang'd previews to appear

I even tried pasting the .lib4d file to both possible library/browser locations for Cinema only to discover it made two Cycles folders within my content browser, each of which display no material previews.

heyBear-- I have not tried that, such a method was unknown to me. How do I go about moving the preferences folder to a different directory? I'm assuming it's more or less involving some copy/paste action... ?

Open C4D preferences and then at the bottom of the window there should be a button that says "Open Preferences Folder". Once there take the "prefs" folder and move it to your desktop. Then quit and launch C4D again. It should automatically build you new prefs. Not sure if it will work, but it's one of the things I try when nothing else works.

Hey Cyclers,
I just joined the few that doesn't seem to have preview capabilities either of the Cycles 4D starter pack.
I have gone through this thread and tried all suggestions and still just getting the generic C4D preset thumbnails and in the preview window I get a rotating thinker wheel.